Light emitting device

ABSTRACT

The present invention provides a light emitting device which comprises a light emitting element, a mounting board on which the light emitting element is mounted, a metal-made reflector surrounding the side surfaces of the light emitting element on the mounting board, a conductor for electrically connecting the light emitting element with the mounting board, and a sealing resin fitted within the reflector to cover and seal the light emitting element and the conductor. The mounting board includes a metal-made base board, and an insulating board laminated on the base board and formed with a window hole extending therethrough which is larger than the outer periphery of the light emitting element. A mount for carrying the light emitting element thereon is disposed on the base board within the window hole with a clearance defined from side surfaces of the window hole. The conductor straddles the clearance, and electrically connects the wiring pattern formed on the insulating board with the light emitting element and the mount. Then, part of the clearance associated with the area that projects from the conductor to the mounting board is narrower than the rest of the clearance.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2008-151651, filed on Jun. 10, 2008, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an LED device which is a light emittingdevice using an LED chip.

2. Description of the Related Art

In general LED devices, a resin is used to seal an LED chip, which is alight emitting element, from the surroundings for purposes of protectingthe LED chip. Also, from the fact that a high-power LED device for usein illumination applications self generates a large amount of heat whenit is turned on, a metal is used for a board and a reflector in order toradiate this self-generated heat (see JP-2001-85748-A).

FIG. 1 is a vertical sectional view showing the configuration of an LEDdevice described in JP-2001-85748-A. Referring to FIG. 1, this LEDdevice comprises LED chip 1, board 2 on which LED chip 1 is mounted, andreflector 3 including a frame which surrounds side surfaces of LED chip1 on board 2.

Then, LED chip 1 mounted on board 2, and bonding wire 4 whichelectrically connects board 2 with LED chip 1 are covered with sealingresin material 5. Sealing resin material 5 is fitted in the frame whichforms part of reflector 3. Reflector 3 is made of a metal material suchas Al, Cu and the like.

In such a configuration, the coefficient of linear expansion in thesealing resin and reflector are largely different from each other. As aresult, when the sealing material for the LED chip is an epoxy-basedresin, the sealing material is highly likely to crack due to the thermalexpansion of each component member while the LED is turned on, orpeeling is highly likely to occur on the interface between the sealingresin and LED chip or on the interface between the sealing resin andreflector, because of the high hardness of the sealing materialexhibits.

Thus, as general countermeasures to the above problem, an elasticsilicone-based sealing resin is employed to avoid cracks and interfacialpealing even if each component member thermally expands.

As described above, when a silicone-based resin is used for the sealingmaterial for the LED chip in the configuration of FIG. 1, the problemsof cracks and interfacial pealing are less likely to occur because thesilicone-based resin exhibits a larger coefficient of linear expansion,as compared with the epoxy-based resin.

However, even if the foregoing problem is solved, this configurationsuffers from another problem in which the bonding wire for supplyingelectric power to the LED chip is pulled and broken.

A mechanism involved in the occurrence of this problem will bedescribed. FIG. 2 is a cross-sectional view which schematically showsthe configuration of the light emitting device in FIG. 1 to describe themechanism involved in the occurrence of the problem. In theconfiguration shown in FIG. 2, a silicone-based resin is fitted inreflector 3 as sealing resin material 5, and this resin covers LED chip1 and bonding wire 4.

Since the light emitting device is configured as described above, whenheat is generated while the LED is turned on, it is anticipated thatsealing resin material 5 fitted within reflector 3 will largely expandin the direction in which it moves away from board 2, and consequentlybonding wire 4 will be pulled and broken (see FIG. 2). This problemcauses a lower reliability of the light emitting device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a light emittingdevice which is capable of solving such problems. An exemplary object ofthe invention is to prevent lower reliability caused by self heatgeneration which occurs while the LED is turned on.

One aspect of the present invention is a light emitting device whichcomprises a light emitting element, a mounting board on which the lightemitting element is mounted, a metal-made reflector surrounding sidesurfaces of the light emitting element on the mounting board, aconductor for electrically connecting the light emitting element withthe mounting board, and a sealing resin fitted within the reflector tocover and seal the light emitting element and the conductor.

The mounting board includes a metal-made base board, and an insulatingboard laminated on the base board and formed with a window holeextending therethrough which is larger than the outer periphery of thelight emitting element.

A mount for carrying the light emitting element thereon is disposed onthe base board within the window hole with a clearance defined from theside surfaces of the window hole.

The conductor straddles the clearance, and electrically connects thewiring pattern formed on the insulating board with each of the lightemitting element and mount.

Further, according to the present invention, part of the clearanceassociated with the projection area of the conductor to the mountingboard is narrower than the rest of the clearance. Alternatively, theconductor is formed in a coil shape, and is retractable. Alternatively,the clearance is filled with a resin which has a lower coefficient oflinear expansion than the sealing resin, and the sealing resin isapplied on the resin. Alternatively, the insulating board has athickness smaller than that of the sub-mount on the side on which thelight emitting element is mounted.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings which illustrate examples of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view showing the configuration of an LEDdevice which is described in JP-2001-85748-A;

FIG. 2 is a cross-sectional view which schematically shows theconfiguration of the light emitting device in FIG. 1 to describe amechanism involved in an occurrence of the problem;

FIG. 3A is a top plan view of a light emitting device according to afirst exemplary embodiment of the present invention, and FIG. 3B is aschematic cross-sectional view of the light emitting device;

FIG. 4A is a partially enlarged view of FIG. 3, and FIG. 4B is across-sectional view of FIG. 4A;

FIG. 5 is a schematic cross-sectional view of a light emitting deviceaccording to a second exemplary embodiment of the present invention;

FIG. 6 is a top plan view of the light emitting device according to thesecond exemplary embodiment of the present invention;

FIG. 7 is a schematic cross-sectional view of a light emitting deviceaccording to a third exemplary embodiment of the present invention;

FIG. 8 is a schematic cross-sectional view of a light emitting deviceaccording to a fourth exemplary embodiment of the present invention; and

FIG. 9 is a schematic cross-sectional view of a light emitting deviceaccording to a fifth exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In the following, exemplary embodiments of the present invention will bedescribed with reference to the drawings.

FIG. 3A is a top plan view of a light emitting device according to afirst exemplary embodiment of the present invention, and FIG. 3B is across sectional view taken along line X-X′ in FIG. 3A.

As shown in these figures, the light emitting device of this exemplaryembodiment comprise LED chip 11, mounting board 12 on which LED chip 11is mounted, and metal-made reflector 13 which is a frame for surroundingside surfaces of LED chip 11 on mounting board 12.

Mounting board 12 includes metal-made base board 14, and insulatingboard 15 laminated on base board 14.

Reflector 13 has an inner wall surface which inclines in a bowl shape toform a light reflection surface.

Materials for base board 14 and reflector 13 may be metal materialswhich exhibit relatively high thermal conductivity, and Al, Cu and thelike are employed. On the other hand, a glass epoxy board is employed asmaterial for insulating board 15.

Rectangular window hole 16, which is larger than the outer periphery ofLED chip 11, extends through insulating board 15, and LED chip 11 ismounted on base board 14 inside of window hole 16 through sub-mount 17.

Further, wiring pattern 21 is routed to make an electrical connection toLED chip 11 on the sides of insulating board 15 and sub-mount 17opposite to base board 14. Then, wiring pattern 21 on insulating board15 is covered with resist 22 for protection.

AIN is employed as a material for sub-mount 17 because it exhibitsrelatively high thermal conductivity and insulating properties.

Then, LED chip 11 mounted on sub-mount 17, and bonding wires 18, whichare conductors made of Au, Al or the like for electrically connectingwiring pattern 21 on insulating board 15, LED chip 11, and sub-mount 17with each other, are covered with sealing resin material 19 made of asilicone-based resin.

Sealing resin material 19 fits within reflector 13. Reflector 13reflects light radiated from LED chip 11.

In the light emitting device having the configuration described above,metal materials are used for base board 14 and reflector 13, and asilicon-based resin having a higher elasticity than an epoxy-based resinis used for sealing resin material 19. Since the silicone-based resinhas a larger coefficient of linear expansion than the relatively hardepoxy-based resin, a larger difference in the coefficient of linearexpansion is present between mounting board 12 and reflector 13 and thesealing resin, as compared with the case where an epoxy-based resin isused for the chip sealing material.

Since sub-mount 17 is placed on base board 14 inside of window hole 16of insulating board 15, window hole 16 is formed large enough toaccommodate the outer periphery of rectangular LED chip 11 therein. As aresult, clearance 23 is formed between the side surface of window hole16 of insulating board 15 and the side surfaces of sub-mount 17. Thus,when LED chip 11 and bonding wire 18 are covered with sealing resinmaterial 19 made of silicone-based resin, this clearance 23 is alsofilled with sealing resin material 19.

Bonding wires 18 straddle clearance 23 in order to electrically connectthe wiring pattern on insulating board 15 with LED chip 11 and sub-mount17.

The aforementioned sealing resin material 19 fits within reflector 13,and when heat is generated while the LED is turned on, sealing resinmaterial 19 expands in a direction in which sealing resin material 19moves away from mounting board 12. In this event, bonding wires 18 arepulled.

Due to the foregoing factor, as an amount of sealing resin material 19that expands becomes larger and larger, bonding wire 18 is more likelyto break.

The following countermeasures are taken such that this problem will notarise in the light emitting device of this exemplary embodiment.

In this exemplary embodiment, as shown in FIGS. 4A and 4B, in clearance23 between the side surface of window hole 16 of insulating board 15 andthe side surfaces of sub-mount 17, clearance 23 a straddled by eachbonding wire 18 is narrower than the rest of clearance 23. Statedanother way, in clearance 23 described above, when bonding wires 18 areprojected onto mounting board 12, the spacings of the clearances nearthe wire projected regions are made narrower than the rest of clearance23. FIG. 4A is an enlarged top plan view showing the surroundings ofclearance 23 in FIG. 3A, and FIG. 4B is a schematic cross-sectional viewtaken along line Y-Y′ in FIG. 4A.

In the example shown in FIG. 4A, a side wall of a portion of window hole16 straddled by each bonding wire 18 is formed to extend convexly towardsub-mount 17. Instead of this shape, the side wall of sub-mount 17associated with each bonding wire 18 may extend convexly toward the sidewall of window hole 16.

In the configuration of this exemplary embodiment, the amount ofexpansion (amount of movement) of sealing resin material 19 at theposition of bonding wire 18 is calculated by the product of the distancefrom mounting board 12 to bonding wire 18 having the coefficient oflinear expansion of sealing resin material 19.

In the configuration as shown in FIG. 4A, with the formation of theconvex extension, the distance between bonding wire 18 and mountingboard 12 becomes shorter at more locations than when the extension isnot formed.

As a result, an amount of sealing resin material 19 that expands at theposition of bonding wire 18 decreases, as compared with a configurationwithout the extension, thus making bonding wire 18 less susceptible tobreak.

Next, another exemplary embodiment will be shown. Here, the samecomponents as those in the configuration shown in FIGS. 3A and 3B aredesignated the same reference numerals with the intention of avoidingrepeated descriptions, and different components will be mainlydescribed.

As shown in FIG. 5, in a second exemplary embodiment, binding wires 18electrically connect between sub-mount 17 and insulating board 15 andbetween LED chip 11 and insulating board 15. Bonding wires 18 are placedcloser to mounting board 12.

This configuration can be made by changing the settings for a movablerange of a capillary in a wire bonder or by collapsing the bonding wiresafter the wires have been connected between pads.

As the distance from mounting board 12 to bonding wire 18 is shorter andshorter, an amount of sealing resin material 19 that expands at thepositions of bonding wires 18 decreases, thus making bonding wires 18less susceptible to break.

In this second exemplary embodiment, window hole 16 may be in the sameshape as the outer peripheral shape of LED chip 11, as shown in FIG. 6.However, this exemplary embodiment can further improve reliability ofthe light emitting device by combining the same with the structure ofthe first exemplary embodiment.

As shown in FIG. 7, in a third exemplary embodiment, resin 20 whichexhibits a lower coefficient of linear expansion than sealing resinmaterial 19 is filled in the clearance between the side surface ofwindow hole 16 of insulating board 15 and the side surfaces of sub-mount17. Further, on the top of insulating board 15, LED chip 11 and bondingwires 18 are covered with sealing resin material 19 made of asilicone-based resin. Resin 20 preferably has a coefficient of linearexpansion as close as possible to the coefficient of linear expansion ofinsulating board 15 and sub-mount 17.

In such a configuration, with resin 20 filled in the clearances, sealingresin material 19 that occupies from bonding wires 18 to mounting board12 will become a smaller volume than the case where only sealing resinmaterial 19 is applied on the surface of insulating board 12 includingthe clearances. As a result, an amount of sealing resin material 19 thatexpands at the positions of bonding wires 18 decreases, as compared withonly sealing resin material 19 applied on mounting board 12, thus makingbonding wires 18 less susceptible to break.

Likewise, in this exemplary embodiment, window hole 16 may be in thesame shape as the outer peripheral shape of LED chip 11, as shown inFIG. 6. However, the third exemplary embodiment can further improve thereliability of the light emitting device by combining the same with thestructure of the first exemplary embodiment or the second exemplaryembodiment or both the first exemplary embodiment and the secondexemplary embodiment.

As shown in FIG. 8, in a fourth exemplary embodiment, insulating board15 has a thickness smaller than that of sub-mount 17. Stated anotherway, the surface of insulating board 15 on which LED chip 11 is mountedis positioned closer to base board 14 than the surface of sub-mount 17on which LED chip 11 is mounted.

Structurally, in the light emitting device of the present invention,distance A from base board 14 which is exposed in the clearance betweeninsulating board 15 and sub-mount 17 to bonding wire 18 in the verticaldirection is different from distance B from insulating board 15 tobonding wire 18 in the vertical direction.

When sealing resin material 19 expands in the direction in which itmoves away from mounting board 12 due to heat generated while the LED isturned on, bonding wires 18 are more likely to suffer from problems ofbreak and peeling because a larger difference between distance A anddistance B results in a larger difference in the amount that movesbetween wire portions associated with respective distances A, B.

Accordingly, in the example shown in FIG. 8, the top surface ofinsulating board 15 is positioned lower than the top surface ofsub-mount 17 in order to reduce the distance between distance A anddistance B the greatest possible amount.

Further, the top surface of insulating board 15 is preferably positionedat substantially an equal level or lower to the top surface of sub-mount17 such that light emitted from the side surfaces of LED chip 11 mountedon sub-mount 17 is not prevented by the side surface of insulating board15. The exemplary configuration of FIG. 8 satisfies such a requirementas well.

Again, in this exemplary embodiment, window hole 16 may be in the sameshape as the outer peripheral shape of LED chip 11, as shown in FIG. 6.However, the fourth exemplary embodiment can further improve thereliability of the light emitting device by combining the same with thestructure of one or more exemplary embodiments from among the first,second, and third exemplary embodiments.

As shown in FIG. 9, in a fifth exemplary embodiment, bonding wires 18themselves are formed in a coil (spring) shape.

In such an exemplary embodiment, bonding wires 18 themselves areretractable. Therefore, even if sealing resin material 19 expands due toheat generated while the LED is turned on in the direction in which itmoves away from mounting board 12, bonding wires 18 expand and are notbroken.

According to each exemplary embodiment as described above, it ispossible to further reduce a stress applied to the bonding wires due toself heat generation while the LED is turned on.

While preferred embodiments of the present invention have been describedusing specific terms, such description is for illustrative purposesonly, and it is to be understood that changes and variations may be madewithout departing from the spirit or scope of the following claims.

1. A light emitting device comprising: a mounting board; a lightemitting element mounted on said mounting board; a reflector surroundingsaid light emitting element on said mounting board; a conductor forelectrically connecting said light emitting element with said mountingboard; and a sealing resin fitted within said reflector to cover andseal said light emitting element and said conductor, wherein saidmounting board includes a metal-made base board, and an insulating boardlaminated on said base board and formed with a window hole which islarger than the outer periphery of said light emitting element, a mountfor carrying said light emitting element thereon is disposed on saidbase board within said window hole with a clearance defined from sidesurfaces of said window hole, said conductor straddles the clearance,and electrically connects a wiring pattern formed on said insulatingboard with each of said light emitting element and said mount, and partof the clearance associated with an area that projects from saidconductor to said mounting board is narrower than the rest of theclearance.
 2. The light emitting device according to claim 1, wherein aside wall of said window hole associated with said conductor is formedto extend convexly toward a side wall of said mount.
 3. The lightemitting device according to claim 1, wherein said conductor is formedin a coil shape and is retractable.
 4. The light emitting deviceaccording to claim 1, wherein said clearance is filled with a resinhaving a lower coefficient of linear expansion than said sealing resin,and said sealing resin is applied on said resin.
 5. The light emittingdevice according to claim 1, wherein a surface of said insulating boardopposite to said base board is positioned closer to said base board thana surface of said mount opposite to said base board.
 6. The lightemitting device according to claim 1, wherein said sealing resin is asilicone-based resin.